Eight extremes: The densest thing in the universe

At the modest temperatures and pressures of Earth’s surface, the densest known material is the metallic element osmium, which packs 22 grams into 1 cubic centimetre, or more than 100 grams into a teaspoonful. Even osmium is full of fluff, however, in the form of electron clouds that separate the dense atomic nuclei. Although rarefied, these clouds are robust, and even the immense pressures deep within the planet can only compress solid matter to a modest degree.

Far greater pressure is found within the collapsed core of a giant star, a remnant we know as a neutron star. There, matter is in some exotic and ultra-dense form – most probably neutrons, and possibly a few protons and electrons, packed cheek-by-jowl. One cubic metre of “neutronium” matter from the centre of a neutron star could have a mass of up to 1018 kilograms, or a million billion tonnes.

An even denser hypothetical material may yet exist in the cores of neutron stars&colon; quark matter, in which protons and neutrons dissolve into their constituent particles. The latest evidence is against it, though. Two newly discovered neutron stars are so heavy that they would probably squeeze a quark-matter core into oblivion. The clues to what really lies at the heart of a neutron star may come through studying starquakes, the juddering explosions of energy that happen when the crust of a neutron star ruptures.

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Neutronium, or perhaps quark matter, may be the densest form of matter in the cosmos, but it is probably …